Multiple coating apparatus



Oct. 24, 1961 .1. F. PADDAY 3,005,440

MULTIPLE COATING APPARATUS Filed Jan. 8, 1959 MTH F l'g. 4

W JOHN E PADDAY INVENTOR.

BYMM

nited Sores arent f n, fil-,005,440` Ice Patented oct. 24, 1961 New Jersey Filed Jan. 8, 1959, Ser. No. 185,713 2 Claims. (Cl. HSR-412) The present invention relates to coating apparatus and particularly to an improved multi-slide coating hopper.

A multi-slide hopper is an apparatus that will simultaneously coat two or more liquids onto a solid support in such a way that the layers Vare not mixed and are individually of uniform thickness. Such a hopper is disclosed in U.S. Patent 2,761,419y and the present invention relates to lan improvement in such hoppers which overcomes a decided shortcoming of known hoppers of this type.

The conventional slide hopper performs its coating operation by metering a rst coating liquid from a supply through a narrow distributing `slot which distributes the liquid uniformly across the top of a downwardly inclined slide surface. This layer of liquid moves down the slide surface hy gravity so as to supply an evened out and steady supply to a coating bead across which the -web to be coated is moved to pick up a layer of liquid therefrom. A second liquid is supplied to, and distributed by, a second distributing slot which in turn directs a uniform layer of the liquid onto the top of a second slide surface so as to ow down to the coating bead, first alone on its own slide surface, and then onto the top of the layer of liquid issuing from the first distributing slot and then down to the coating bead in superposcd relation with the layer of rst coating liquid. Subsequent liquids may be coated simultaneously by equipping the hopper with the appropriate number of distributing slots and slide surfaces.

It has been found that the conventional slide hopper often tends t produce a `defect in the nal coated product which appears as a long line or lines running parallel to the direction of coating. These defects are not always visible in the product as coated and vvery often they become visible only after the product is dried :and/or processed (if the coated web is a photographic product) and then is visually checked. Experimentation has shown that these lines, which will hereinafter be referred to as pencil lines, are the result of long lines which are locally deficient in the coating liquid issuing from the lowermost slot, where a two-slot hopper is used, and which arev thicker in the second coating liquid, the tot-al thickness of the two liquids being the same as the unaffected partsy of the coatings.

I have found that these pencil lines are caused by a blockage in the lowermosty distributing slot, or in the slots where two layers of liquid come together in moving down the slides to the coating bead. These pencil lines can be avoided either by removing ,the cause of the blockage. in the distributing slots, i. e., taking out gelatin slugs, or unusually large silver grains when silver halide sol-utions are being coated, etc., or -by redesigning a part of the hopper so that an interruption of -flow due to such `a blockage does not result in a pencil line.

The primary object of the present invention is to modify the design ofl the conventional slide hopper having two or more slides so -as to enable coatings to be made which are cfree from pencil lines even when distributing slot `blockages, occur.,

Anotherobject of the present invention is to widen out the exit end of the discharge slots where two* liquids come into contact .in such a way as to produce a turbulence the stream of liquid passing through the slot by reason of a sudden change in velocity and before it leaves the slot. This turbulence willoause any pencil line formed in the stream 'by reason of a blockage in the narrow part of the slot to heal itself -before the stream issues fromr the slot and onto its slide surface where it is immediately contacted by a second layer of liquid moving down upon it fromV a higher slide surface.

And still another object is to widen the exit end of the distributing slots by cutting back the surfaces of the narrow portion 'of the slot at'right angles so that a sudden enlargement ofthe s'lot is achieved. This sharpV right-angle corner produces the maximum amount of turbulence in the stream and thereby produces the most effective healing action for any pencil lines formed in the stream by reason of bloclizagesV in the narrow portion of the slot.

The novel features -that I consider characteristic of my invention are set forth with particularity Yin the appended claims. The invention itself, however, vboth as to its details of' construction and its methods of operation, together with additirmal objects andadvantages thereof, will lbest be understood'fom Vthe vfollowing description when read in connection withthe accompanying drawings, in which: i i i FIGA is a sectional detail of -a dual hopper constructed in accordance with a preferred embodiment of the present invention;

FIG. 2 is an enlarged sectional detail showing the exit end of .the lowermost distributing slot enlarged on the down yside ofthe slot in accordance with one embodiment of my invention;

FIG. 3 is similar -to FIG. 2, `Ibut showing the exit end of the lowermost discharge slot enlarged on the upside of the slot; and

FIG. 4 is an enlarged transverse sectional detail of a web coated with a conventional dual slide hopper and illustrating, in v:an exaggerated way, how objectional pencil lines might appear in the iinal product.

In FIG. l my'invention is shown on a conventional dual slide hopper of the type disclosed in U.S. Patent 2,761,419. With this hopper, one fluid coating composition C1' is continuously pumped by a constant discharge metering pump P into a cavity 10 at 4a given rate through inlet 11 and from which it is forced'through `a narrow vertical distributing slot 12 in the'form of a ribbon and out onto a downwardly inclined slide surface 13 down which it flows by gravity in the -formof ,a layer 14 .to a point where it forms a coating bead B 'between the lip 15 of the slide surface 13 yand a web W moved upwardly across and in contact with the bead by a supporting roll 16.

The second coating composition C2 is continuously pumped into asecond cavity 1'7v lat a given rate by another constant ldischarge metering pump P through inlet i8 and from which it isv forced through a narrow vertical distributing slot v19 in the form of a ribbon and out onto a second downwardly inclined -slide surface 20. This ribbon ofthe second coating composition in flowing down the slide surface 20 under theinluence of gravity =forms a smooth and uniformly thick layer 21. The t-wo slide surfaces 13 and 20 are coplanar, or substantially so, so that as the layer 21 of the second coating composition reaches the ribbon of the first coating composition issuing from slot 12, it ilows up on top of the same and the two then diow together down slide surface 13 and into the coating -bead B. The relative thickness of the two lilyers of coating materials will depend upon the rate at which they are Pumped into the respective cavities 10; and 1'7. The hOlJper is provided with pipes 2.3 through which hot or cold uids can be circulated to keep the' coating compositions in a desired fluid state. The slide hopper and the method of coating carried out by the use thereof described up to this point is shown in the abovenoted U.S. Patent 2,761,419, and the improvement therein which constitutes the present invention will now be described.

The purpose of the distributing slots 12 and 19 is to cause the issuance or extrusion of the liquid coating compositions in a uniform ribbon across the entire width of the hopper or theY length of thedistributing slots. Since the hopper may b e as wide as 48" to 54" and the cavities and 17 for the coating compositions do not extend the full length of the slots, but only over a limited length of the center portion thereof, these distributing slots must be quite narrow in order to do a satisfactory iob of distributing the coating compositions throughout the full length of the slots. The length and depth of these distributing slots may vary with the solutions to be coated, the uniformity in thickness required over the full length of the slot, and the iiow rates to be used. At best they are very narrow, e.g., .009 or .010 inch. As pointed out above, I have found that pencil lines often appeared in the coated product made with these slide hoppers. In FIG. 4 an attempt has been made to illustrate such pencil line imperfections and their character. Looking at FIG. 4, which is a partial and greatly enlarged transverse cross-section of a web coated with a dual slide hopper, it will be seen that the layer 14 of coating composition C1 directly engaging a web W has two indentations 30 running lengthwise of the web which are 4iilled with the second coating solution forming layer 21. It will thus be appreciated that at the indentations 30 the thickness of layers 14 and 21 will be relatively different than at other portions of the web notwithstanding the fact that the over-all physical thickness of the two layers at these particular points may be the same as at all other points across the web. Obviously then these pencil lines cannot be readily physically measured but will be visually apparent after drying or processing if one of the layers is a silver halide solution and the other is a clear gelatin solution as found in photographic iilm. I have found that these pencil lines occur by reason of a small obstruction in the distributing slot 12, e.g., a grain of silver in the silver halide solution or a slug of geltain in a photographic emulsion, which causes an interruption in the dow pattern moving out of slot 12 and onto the slide surface 13. In a single layer exposed to the air this is self-healing due to surface forces tending to reduce the total exposed surface to a minimum. However, when the layer 21 of the second coating composition C2 is run onto the rst as it issues from slot 12, then the deficiency, or indentations 30, in layer 14 is lled up with the second coating composition, and providing the interfacial tension between coating composition C1 and composition C2 is small or zero, then a pencil line is formed, as shown at 30 in FIG. 4, because there is healing force at their interface. The top surface of layer 21 soon heals due to its surface tension.

These pencil lines can be avoided either by removing the cause of the blockage, e.g., taking out geltain slugs where the coating composition contains gelatin or removing excessively large dispersed particles where the coating solution is a dispersion, or by redesigning a part of the hopper so that an interruption of flow of this nature does not result in pencil lines.

The present invention relates to a modification in the hopper design, where it has two or more slides, so as to enable coatings to be made free of pencil lines even when slot blockages occur.

In the dual hopper shown in FIG. 1 this modification in design consists in enlarging the exit end of the distributing slot 12, as clearly shown at 35 in FIGS. 2 and 3, so that obstructions are not likely to interrupt flow in the critical position, making the enlargement sudden and suiciently wide so as to produce turbulence and thereby heal any interruption of liiow that may occur as the re- Y other dispersions.

sult of a blockage, and making the depth of the enlargement suiiicient to allow laminar flow to be regained after the initial turbulence. This enlargement may be made either by cutting away the lower side of the slot 12, as shown in FIG. 2, or by cutting away the upper side of the slot 12, as shown in FIG. 3. The enlargement of the exit end of slot -12 should be made sudden and sufficiently wide so as to produce sufficient turbulence in the issuing stream of coating composition to heal interruptions of iiow due to blockages in the narrow portion of slot 12 under all conditions likely to be encountered when coating photographic emulsions and/ or To this end and the enlargement should be cut back at right angles to the surface of the slot to form a sharp corner 36. Now if an obstruction occurs in the narrow portion of slot 12 and forms an interruption in the ow of the ribbon of coating composition C1, when this ribbon of liquid reaches an en- -larged portion 35, a turbulence will be set up in the flow to heal the interruption before it leaves the slot and passes onto the slide surface 13. No obstruction will occur in the enlarged end of the slot 12 where turbulence will not heal the interruption it might cause because any particle which can pass through the narrow portion of the slot 12 will obviously pass through the enlarged portion 35 of the slot. The back corner 37 of the enlargement 35 may be provided with a small radius to facilitate cleaning. The depth of the enlargement 35 should be suiiicient to allow laminar flow to be regained after the initial turbulence and prior to the time the ribbon of coating composition issues onto the slide surface 13. The minimum depth of the enlargement depends upon the ow rates and viscosity of the coating solution used, and is more or less proportional to the flow rate. Low viscosity coating compositions andglow flow rates would normally require less depth than high viscosity solutions and high -flow rates. The width of the distributing slot 12 is chosen such as to obtain a uniform thickness of coating solution over the full length of Ithe exit end of the slot. If a relatively wide slot is used, it must be deep, whereas a narrow slot can be less deep to obtain the desired uniform distribution over the entire length of the eXit end of the slot.

With a distribution slot of 0.009", eifective working has been obtained using an enlargement of from 0.015" to 0.005" wide and a depth of 0.150". A hopper was constructed from a transparent plastic material in the manner shown in FIG. =1, but with the exit end of the distributing slot 12 modified as clearly shown in FIG. 2. Artificial blockages were introduced at points both in the modiiied and in the conventional slot. When gelatin and sodium alginate solutions of equivalent viscosity and ow rates were passed through the hopper, pencil lines were seen only behind the block-ages in the original design. The modiiied slots, although blocked, showed no pencil lines.

While I have disclosed my invention as applied to a dual slide hopper, it follows that it could also be used on a hopper having three, four or more slides. In these cases, any or all distribution slots below any slide surface, or where two layers are brought into contact, will be enlarged at their exit ends, as shown at 35 in FIGS. 2 and 3, to prevent the formation of pencil lines.

While I have shown and described specific embodiments of my invention, I am aware that many modifications thereof are possible. My invention, Itherefore, is not to be limited to the precise details shown and described but is intended to cover all modifications coming within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. A multiple-layer hopper for feeding a plurality of uid coating compositions in superposed and distinct layer relationship to a point of deposition where they are simultaneously deposited onto the surface of a web moving relative thereto in such strata relationship, and comprising a block, a slide surface on said block inclined down- Wardly and terminating in a lip adjacent said point of deposition, a plurality of narrow distributing slots, one for each coating composition, extending through said block to said slide surface through which a thin layer of coating composition is adapted to be discharged onto said slide surface, the lowermost of said discharge slots spaced from said lip and the remaining distributing slots spaced above this lowest slot and from each other to provide an uninterrupted slide portion below each slot over which the layer of coating solution discharged therefrom flows by gravity to form a smooth layer of uniform thickness before, in the case of all but the lowest slot, it reaches the next lowest discharge slot and ows onto the top of, and along with, the layer of coating composition discharged therefrom, and, in the case of the lowest discharge slot, before it reaches the lip; the discharge end of at least one of said lowermost of said slots being wider than the major portion of the slot and the enlargement in the slot being sudden and suiciently great so as to produce a turbulence in the coating composition iiowing through the Wider portion of the slot and thereby heal any interruption of ow that may occur as the result of a blockage in the narrow portion of the slot.

2. A multiple-layer hopper according to claim 1, characterized by the fact that the wider discharge end of the distribution slot and the narrow portion of the slot are joined by a right-angle corner.

References Cited in the tile of this patent UNITED STATES PATENTS 1,947,791 Moyer ....v--.... Feb. 20, 1934 2,309,981 Randell .J Feb. 2, 1943 2,761,419 Mercier et a1. Sept. 4, 1956 2,784,697 Uhleen f Mar. 12, 1957 

